1. Field of the Invention
The present invention relates to an optical scanning device, light screening material, method for screening a flare light, and image formation apparatus.
2. Description of the Related Art
Conventionally, in an optical scanning device installed on an electrophotographic image formation apparatus such as a laser printer, copier, and the like, a light beam irradiated from a light-beam irradiation device including a laser light source in accordance with an image signal is periodically deflected using a deflector including a rotating polygon mirror (polygon mirror), for example. The deflected light beam is imaged on a scanning surface of a photosensitive recording medium (photoconductor drum) using a scanning optical system. Image recording is performed from the image.
Specifically, FIG. 1 is a schematic plan view showing a conventional optical scanning device. In FIG. 1, a light beam emitted from light-beam irradiation devices 101 to 104 is irradiated onto a deflection surface (reflection surface) of a deflector 105 such as a polygon mirror. The light beam reflected and deflected on the deflection surface of the deflector 105 is guided on scanning surfaces of photoconductor drums 108 and 111 via scanning optical systems 109 and 110. By rotating the deflector 105, it is possible to perform optical scanning on the scanning surfaces of the photoconductor drums so as to transmit and record image information. In FIG. 1, four light beams are irradiated onto the deflector 105 and four deflected light beams are used for optical scanning. However, two pairs of scanning optical systems are overlapped each other, so that only two scanning optical systems are illustrated. In this case, in each scanning lens of the scanning optical systems disposed in an opposing manner, a portion of projected light beam is reflected on a surface of the lens and scattered as a flare light. When the flare light is projected onto the scanning lens of the opposite scanning optical system, the light is guided to the scanning surface on the opposite side via the scanning optical system and noise is generated on image information to be recorded, so that a streak stain or a ghost image is generated on a formed image. Or, a ground stain or blur is generated in accordance with covering with the flare light. This would cause degradation of image quality. In some cases, plural scanning lenses are disposed in each scanning optical system. However, in the present invention, the scanning lens refers to a scanning lens onto which the light beam from the deflector is first projected unless otherwise specified.
In view of this, there is known an antireflection film deposited on a surface of the scanning lens so as to reduce the flare light. Further, there is a technique of providing a light screening function to a case for controlling noise accompanied by high-speed rotation of the polygon mirror, in which the case is made opaque except windows for light projected onto the polygon mirror and light reflected on the polygon mirror. However, in proportion as an intense light beam is used and high-quality image formation is required, even a slight amount of flare light would cause considerable degradation upon image formation.
In order to prevent such a flare light from entering the opposite scanning lens, a light screening material is disclosed (refer to Patent Document 1). FIG. 2 shows an example. Although FIG. 2 shows an optical scanning device almost the same as in FIG. 1, in FIG. 2, two light screening materials 89 and 90 are disposed in the vicinity of a deflector 62. The light screening materials 89 and 90 each screens flare lights with both surfaces, the flare lights being reflected from scanning lenses 63 and 64 disposed in an opposing manner. For example, in the light screening material 89, the flare light reflected from the scanning lens 63 is screened with a right side surface in the figure and the flare light reflected from the scanning lens 64 is screened with a left side surface in the figure. In this manner, it is possible to screen flare lights of four locations using two light screening materials and to efficiently screen the flare light.
Patent Document 2 discloses an optical scanning device in which the light screening material for screening the flare light is disposed on a non-active area (area not used as an optical path of a light beam) so as to prevent the flare light from entering the opposite scanning lens, the non-active area being positioned between an optical path of light flux projected on the deflection surface of the deflector and an optical path of scanning light flux deflected on the deflection surface.
Patent Document 1: Japanese Laid-Open Patent Application No. 2002-196269
Patent Document 2: Japanese Laid-Open Patent Application No. 2003-202512
As mentioned above, optical scanning devices provided with a light screening material for screening the flare light have been proposed and an effect of screening the flare light has been sufficiently acknowledged. However, when the light screening material is disposed in the vicinity of the deflector as mentioned above, a sound of wind noise accompanied by rotation of the deflector is increased and this poses a problem in some cases. Such a sound of wind noise from the deflector has been problematic and a case of the deflector, for example, has been used so as to reduce the sound. However, when the light scanning material is disposed closely to the deflector, this light scanning material functions as a baffle board and the sound of wind noise is likely to be increased. However, in the methods disclosed in Patent Documents 1 and 2, unless the light screening material is disposed in the vicinity of the deflector, the effect of screening the flare light becomes insufficient. In view of this, there have been demands for developing an optical scanning device having the light screening material sufficiently capable of screening the flare light or an optical scanning device with a light screening structure without increasing the sound of wind noise.